Extremely slow counterion dynamics in xanthan liquid crystal through ²³Na and ¹⁴N NMR

To describe the structure and dynamics in a lyotropic xanthan liquid crystal, we have done ²³Na and ¹⁴N NMR experiments. The spectra are consistent with a powder average over randomly oriented liquid-crystalline domains. From ¹⁴N quadrupolar echo experiments it was concluded that the exchange among the several micron sized domains is slow with residence times in the millisecond range. Conventional spin-lock experiments (¹⁴N and ²³Na) as well as spin-lock experiments filtered through the quadrupolar spin polarization order (²³Na only) revealed the existence of an extremely low frequency, submegahertz dispersion in the spectral densities pertaining to both sodium and ammonium counterions. The corresponding correlation times are on the order of tens of microseconds and agree with a structural correlation length on the order of 500 nm. The latter correlation times reflect the rearrangement of counterions among polymer segments over the same spatial extent as the polymer persistence length.